University of Ljubljana, Faculty of Mechanical Engineering

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The Faculty of Mechanical Engineering (FME) is a member of the University of Ljubljana, which is, with more than 50,000 students, the third biggest university in Europe. The FME has a more than 60 years long tradition both academically as well as in terms of research and expertise, and has developed a national and international reputation. The Ljubljana FME aims at creating and disseminating knowledge that will enable its students to join the international scientific scene and make its research partners competitive on the global market. It has a clear vision to become the leading teaching and research institution for mechanical engineering in Central and Southeast Europe while maintaining the highest educational and professional standards.

The FME has strong links with Slovenian industry, and is a partner in a number of international R&D projects, and this through intensive cooperation in research clusters, centres of excellences and technology networking. There are presently 13 R&D programme groups, which completed the following research projects in the year 2010: 295 Industrial projects, 39 Applied projects, 5 Basic research projects, and 49 International projects.

Project 1. Wind flow prediction over complex terrain

In this project wind effect on a structure was examined with various wind directions. The structure is situated on a complex terrain. Ansys Fluent was utilized in order to accurately solve turbulence structures in complex terrain. Objective of this research was to analyze the wind velocity effect of erecting barrier in front of the building. Velocity profiles were correctly predicted and optimum barrier configuration was designed. (LECAD-Laboratory for Engineering Design)

Project 2. Improvement of centrifugal pump performance

Objective of this project was performance improvement of existing centrifugal pump by changing impeller’s geometry. In order to do so, profound understanding of flow dynamic was necessary. Ansys Fluent was used to analyze and assess fluid flow dynamics in centrifugal pump. Velocity and pressure profiles were analyzed. Goal of this project was to increase pump’s efficiency. Improved model has pump efficiency by 10% higher from the original impeller design. (LECAD-Laboratory for Engineering Design).

Project 3. Improvement of aerodynamic characteristics of a bike

Aim of this project was bike performance improvement. Improvement was achieved by changing the shape of the bike in order to improve aerodynamic characteristics, in particular drag coefficient. Ansys Fluent was used to explore complex flow around the bike. Good agreement between numerical data and experimental measurements were attained. (LECAD-Laboratory for Engineering Design).

Project 4. Advanced ventilation for airborne cross-infection control in spaces

To prevent the spread of pathogens or pathogen-drops in a space with direct exposure to exhaled air between two facing each other seated persons, we have developed advanced methods of local ventilation. Experimental analysis using thermal manikin have proven that the corresponding air jet can significantly reduce the exposure and the likelihood of transmission of infection in the case of breathing out, talking, and coughing through the free room air. Experimentally, it has been determined the protective efficacy of geometrically optimized system of corresponding air jets in the event of a direct pulse of coughing out air for different levels of cough. The measured time-dependent concentrations of exposure are used to validate the CFD numerical model, made using the Ansys Fluent CFD package. The Fluent RNG k-ε turbulence model in combination with a pronounced modelling of flow in the boundary layer near the wall was used. For pressure interpolation the PRESTO scheme was used. Detailed analyses of the spreading of pathogenic drops of different sizes in the case with and in the case without the air jet protection system were done. The developed advanced method has a great potential in the case of direct exposure to different levels of coughing. It offers full protection in the case of an average cough, and about 87 % reduction of the exposure in the case of the strongest experimentally analysed cough. It was found out that in all simulated cases correspondingly powerful jet of air provides effective protection also against major pathogenic drops of the size greater than 12 µm. (Laboratory for Heating, Sanitary, Solar and Air Conditioning Engineering).

Project 5. Optimization of axial flow force in a hydraulic sliding spool valve by using ANSYS CFD simulation tool

Stationary axial momentum forces of the oil flow through the valve, which increase with the increasing flow rate and pressure difference, determine the external actuation forces required for controlling a sliding-spool valve. One of the goals in the research of hydraulic valve development and optimization in the laboratory LASIM is also to enable the direct control of the valves for bigger nominal valve sizes. It is well known that the direct control of hydraulic valves is limited to smaller nominal valve sizes up to NV10, by using conventional electromechanical actuators, like solenoids. This research shows the possibilities of using the direct control of the valves also for higher hydraulic powers and, consequently, the use of the advantages of the direct control also for higher flow rates and pressure differences. This is achieved by the reduction or compensation of the axial flow forces acting on the valve sliding spool in the closing direction. The implemented flow-force investigations and the resulting modifications of the sliding spool and the valve socket prove that it is possible to reduce the flow force in the valve and to eliminate the non-desirable non-linear digressive flow-force characteristics by implementing some geometrical modifications to the sliding spool and the valve socket. All the control edges of the sliding spool are investigated separately. Finally, an optimal spool and spool-valve socket configuration is found and shows, compared with the non-compensated valve, significantly better results with clearly reduced maximum flow forces and with very convenient flow-force characteristics. (Laboratory for Handling, Assembly and Pneumatic).

Project 6. Fluid flow separation in supersonic nozzles

One parameter that controls the supersonic nozzle performance is the nozzle pres¬sure ratio (NPR), which is the ratio between nozzle inlet and ambient pressure. When the supersonic nozzle operates below designed NPR, flow separation can occur inside divergent part of the nozzle. This leads to the decrease in nozzle performance and generation of lateral forces on the nozzle wall. The aim of the work was focused on the numerical determination of fluid flow sepa¬ration inside small conical supersonic nozzle and inside subscale truncated ideal contour (TIC) supersonic nozzle at low NPRs. Accuracy of numerical simulations are dependent on mesh density. To reduce calculation time and to sim¬plify meshing, gradient method for automatic mesh refinement of finite element mesh was developed and implemented into Ansys batch file. The algorithm works on a principle of maximum gradient of fluid variables, e.g. pressure, velocity and density. Axisymmetric turbulent flow of compressible viscous adiabatic fluid was simulated with standard k-ε turbulent model. Results of numerical simulations were in good agreement with published empirical and semi-empirical flow separation models. (Dejan Nožak, Laboratory for Aeronautics).

Project 7. Conceptual design of a single seat multirole turboprop aircraft

Multirole turboprop aircraft are going to be in high demand at different armed forces throughout the world in the near future. They provide multi-mission capability on a low-cost platform with the addition of state-of-the-art sensors, data links, cockpit and aircraft protection components and various weapons capabilities. They are capable of performing missions such as intelligence, surveillance and reconnaissance, light attack including combat search and rescue, close air support and forward air control. Laboratory for aeronautics performed conceptual design for such aircraft. Aerodynamic analysis of the final concept was performed in Ansys CFX. Computational domain was meshed with hexahedral and tetrahedral elements. Aircraft lift and drag coefficients were calculated for various angle of attack positions at three different cruise velocities: vC=200km/h, vC=400km/h and vC=600km/h. Results from aerodynamic analysis were used for aircraft performance calculations. (Dejan Nožak, Laboratory for Aeronautics).

Project 8. Induced drag estimation in upwind yacht sails

One part of the EUREKA Sailpower project was to investigate main aerodynamic characteristics for the main sail of the MELGES 24 class race yacht. Aerodynamic analysis for upwind sailing conditions was performed in Ansys CFX. Computational domain was meshed with hexahedra elements. Main sail lift, induced drag and total drag coefficients were calculated for various angle of attack positions at three different mean wind speeds v=5m/s, v=10m/s and v=15m/s. Headsail and the hull were not considered in the simulations. Main sail was simulated in neutrally stable atmospheric boundary layer which was defined by the mean wind speed at the height of 10 meters. Results of the numerical simulations were total and induced drag and optimum main sail angle of attack for maximum drive force at minimal aerodynamic drag. In these conditions induced drag represents more than 90% of total aerodynamic drag. (Dejan Nožak, Laboratory for Aeronautics).

Project 9. Finite element analysis of sandwich panels with openings

Lightweight structural sandwich panels made of two thin steel facings, which are bonded to relatively thick and light insulating rock wool core, are often used for wall claddings. The sandwich panel structures are frequently weakened by cut-outs and openings for doors, windows, etc. The effects of these large openings on load-bearing capacity and structural stiffness of sandwich panel claddings are studied. The research project was conducted using a series of numerical analyses and laboratory experiments to study the local and global behaviour for a certain sandwich panel product and certain design case. A few typical design examples of claddings with a large opening were studied. Finite element analyses (by Ansys programme) of the simply supported interconnection systems with three sandwich panels and a large rectangular opening were undertaken. Detailed investigation for a case of a large opening in panel interconnecting system with a middle sandwich panel completely cut is presented. In this case, the load transfer from a sandwich panel with a large opening through the longitudinal joints to the adjacent panels is of the main importance. (Laboratory for transportation machinery and load-bearing structures – LASOK).

Project 10. Finite element analysis of cross-country skiing boot torsion stiffness

A 3D FEM model (by Ansys programme) of an existing cross-country skiing boot in combination with the model of a foot has been made with the purpose of determination of the torsion stiffness of individual parts of the boot. For this reason the realistic material properties of the boot parts were defined by means of measurements. It turned out that the front part of the boot contributes approx. 80 % to the whole torsion deformation, which is why the boot components of this part were studied in greater detail. It was further determined that the front part of the soles contributes approximately 44 % of the whole model stiffness, while the shoe-upper around 25 %. If considering the mass contribution the shoe-upper turns out to be more beneficial than the soles. (Laboratory for transportation machinery and load-bearing structures – LASOK).